High strength steel sheet and method for manufacturing the same

ABSTRACT

The invention provides a high strength steel sheet which exhibits excellent chemical convertibility and corrosion resistance after electrodeposition coating even in the case where the steel sheet has a high Si content, and a method for manufacturing such steel sheets. The method includes continuous annealing of a steel sheet which includes, in terms of mass %, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at 0.005 to 0.060% and S at ≦0.01%, the balance being represented by Fe and inevitable impurities, while controlling the dew-point temperature of the atmosphere to become not less than −10° C. when the heating furnace inside temperature is in the range of not less than A° C. and not more than B° C. during the course of heating (A: 600≦A≦780, B: 800≦B≦900).

CROSS REFERENCE TO RELATED APPLICATION

This application is the U.S. National Phase application of PCTInternational Application No. PCT/JP2010/067612, filed Sep. 30, 2010,the disclosure of which is incorporated herein by reference in itsentirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a high strength steel sheet havingexcellent chemical convertibility and corrosion resistance afterelectrodeposition coating even in the case where the steel sheet has ahigh Si content, as well as to a method for manufacturing such steelsheets.

BACKGROUND OF THE INVENTION

From the viewpoint of the improvements in automobile fuel efficiency andcrash safety of the automobiles, there have recently been increasingdemands for car body materials to be increased in strength for thicknessreduction in order to reduce the weight and increase the strength of carbodies themselves. For this purpose, the use of high strength steelsheets in automobiles has been promoted.

In general, automotive steel sheets are painted before use. As apretreatment before painting, a chemical conversion treatment calledphosphatization is performed. The chemical conversion treatment forsteel sheets is one of the important treatments for ensuring corrosionresistance after painting.

The addition of silicon is effective for increasing the strength and theductility of steel sheets. During continuous annealing, however, siliconis oxidized even if the annealing is performed in a reductive N₂ H₂ gasatmosphere which does not induce the oxidation of Fe (which reduces Feoxides). As a result, a silicon oxide (SiO₂) is formed on the outermostsurface of a steel sheet. This SiO₂ inhibits a reaction for forming achemical conversion film during a chemical conversion treatment, therebyresulting in formation of a microscopical region where any chemicalconversion film is not generated. (Hereinafter, such a region will besometimes referred to as “non-covered region”). That is, chemicalconvertibility is lowered.

Among conventional techniques directed to the improvement of chemicalconvertibility of high-Si containing steel sheets, patent document 1discloses a method in which an iron coating layer is electroplated at 20to 1500 mg/m² onto a steel sheet. However, this method entails theprovision of a separate electroplating facility and increases costscorrespondingly to an increase in the number of steps.

Further, patent documents 2 and 3 provide an improvement inphosphatability by specifying the Mn/Si ratio and by adding nickel,respectively. However, the effects are dependent on the Si content in asteel sheet, and a further improvement will be necessary for steelsheets having a high Si content.

Patent document 4 discloses a method in which the dew-point temperatureduring annealing is controlled to be −25 to 0° C. so as to form aninternal oxide layer which includes a Si-containing oxide within a depthof 1 μm from the surface of a steel sheet base as well as to control theproportion of the Si-containing oxide to be not more than 80% over alength of 10 μm of the surface of the steel sheet. However, the methoddescribed in patent document 4 is predicated on the idea that thedew-point temperature is controlled with respect to the entire areainside a furnace. Thus, difficulties are encountered in controlling thedew-point temperature and ensuring stable operation. If annealing isperformed while the controlling of the dew-point temperature isunstable, the distribution of internal oxides formed in a steel sheetbecomes nonuniform to cause a risk that chemical convertibility may bevariable in a longitudinal direction or a width direction of the steelsheet (non-covered regions may be formed in the entirety or a portion ofthe steel sheet). Even though an improvement in chemical convertibilityis attained, a problem still remains in that corrosion resistance afterelectrodeposition coating is poor because of the presence of theSi-containing oxide immediately under the chemical conversion coating.

Further, patent document 5 describes a method in which the steel sheettemperature is brought to 350 to 650° C. in an oxidative atmosphere soas to form an oxide film on the surface of the steel sheet, andthereafter the steel sheet is heated to a recrystallization temperaturein a reductive atmosphere and subsequently cooled. With this method,however, it is often the case that the thickness of the oxide filmformed on the surface of the steel sheet is variable depending on theoxidation method and that the oxidation does not take place sufficientlyor the oxide film becomes excessively thick with the result that theoxide film leaves residue or is exfoliated during the subsequentannealing in a reductive atmosphere, thus resulting in a deteriorationin surface quality. In EXAMPLES, this patent document describes anembodiment in which oxidation is carried out in air. However, oxidationin air causes problems such as giving a thick oxide which is hardlyreduced in subsequent reduction or requiring a reductive atmosphere witha high hydrogen concentration.

Furthermore, patent document 6 describes a method in which a cold rolledsteel sheet containing, in terms of mass %, Si at not less than 0.1%and/or Mn at not less than 1.0% is heated at a steel sheet temperatureof not less than 400° C. in an iron-oxidizing atmosphere to form anoxide film on the surface of the steel sheet, and thereafter the oxidefilm on the surface of the steel sheet is reduced in an iron-reducingatmosphere. In detail, iron on the surface of the steel sheet isoxidized at not less than 400° C. using a direct flame burner with anair ratio of not less than 0.93 and not more than 1.10, and thereafterthe steel sheet is annealed in a N₂ H₂ gas atmosphere which reduces theiron oxide, thereby forming an iron oxide layer on the outermost surfacewhile suppressing the oxidation of SiO₂ which lowers chemicalconvertibility from occurring on the outermost surface. Patent document6 does not specifically describe the heating temperature with the directflame burner. However, in the case where Si is present at a high content(generally, 0.6% or more), the oxidation amount of silicon, which ismore easily oxidized than iron, becomes large so as to suppress theoxidation of Fe or limit the oxidation of Fe itself to a too low level.As a result, the formation of a superficial reduced Fe layer by thereduction becomes insufficient and SiO₂ comes to be present on thesurface of the steel sheet after the reduction, thus possibly resultingin a region which may not be covered with a chemical film.

PATENT DOCUMENT

-   [Patent document 1] Japanese Unexamined Patent Application    Publication No. 5-320952-   [Patent document 2] Japanese Unexamined Patent Application    Publication No. 2004-323969-   [Patent document 3] Japanese Unexamined Patent Application    Publication No. 6-10096-   [Patent document 4] Japanese Unexamined Patent Application    Publication No. 2003-113441-   [Patent document 5] Japanese Unexamined Patent Application    Publication No. 55-145122-   [Patent document 6] Japanese Unexamined Patent Application    Publication No. 2006-45615

SUMMARY OF THE INVENTION

The present invention has been made in view of the circumstancesdescribed above. It is therefore an object of the invention to provide ahigh strength steel sheet which exhibits excellent chemicalconvertibility and corrosion resistance after electrodeposition coatingeven in the case of a high Si content, as well as to provide a methodfor manufacturing such steel sheets.

Conventional approaches were based on simply increasing the water vaporpartial pressure or the oxygen partial pressure in the entire inside ofan annealing furnace so as to raise the dew-point temperature or theoxygen concentration and thereby to produce excessive internal oxidationof a steel sheet. Consequently, as mentioned above, various problemshave been encountered such as difficulties in controlling the dew-pointtemperature or the oxidation, the occurrence of uneven chemicalconvertibility and a decrease in corrosion resistance afterelectrodeposition coating. Thus, the present inventors studied a novelapproach based on an unconventional idea capable of solving the aboveproblems. As a result, the present inventors have found that because adeterioration in corrosion resistance after electrodeposition coatingcan originate from a surface portion of a steel sheet, moresophisticated controlling of the microstructure and configuration of thesurface portion of the steel sheet allows for obtaining a high strengthsteel sheet having excellent chemical convertibility and corrosionresistance after electrodeposition coating. In detail, a chemicalconversion treatment is performed after annealing is carried out in sucha manner that the dew-point temperature of the atmosphere is controlledto become not less than −10° C. when the heating furnace insidetemperature is in a limited range of not less than A° C. and not morethan B° C. during the course of heating (A: 600≦A≦780, B: 800≦B≦900). Inthis manner, selective surface oxidation and surface segregation can besuppressed, resulting in a high strength steel sheet exhibitingexcellent chemical convertibility and corrosion resistance afterelectrodeposition coating. Here, the term “excellent chemicalconvertibility” means that a steel sheet having undergone a chemicalconversion treatment has an appearance without any non-covered regionsor uneven results of the chemical conversion treatment.

A high strength steel sheet obtained in the above manner comes to have amicrostructure and configuration in which a surface portion of the steelsheet extending from the steel sheet surface within a depth of 100 μmcontains an oxide of at least one or more selected from Fe, Si, Mn, Aland P, as well as from B, Nb, Ti, Cr, Mo, Cu and Ni at 0.010 to 0.50g/m² per single side surface, and in which a region extending from thesteel sheet surface to a depth of 10 μl is such that a crystalline Si/Mnoxide has been precipitated in base iron grains that are within 1 μmfrom grain boundaries. Because of this configuration, deterioration incorrosion resistance after electrodeposition coating is realized andexcellent chemical convertibility is obtained.

The present invention is based on the aforementioned findings. Featuresof embodiments of the invention are as described below.

[1] A method for manufacturing high strength steel sheets, includingcontinuous annealing of a steel sheet which includes, in terms of mass%, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001to 1.0%, P at 0.005 to 0.060% and S at ≦0.01%, the balance beingrepresented by Fe and inevitable impurities, while controlling thedew-point temperature of the atmosphere to become not less than −10° C.when the heating furnace inside temperature is in the range of not lessthan A° C. and not more than B° C. during the course of heating whereinA is 600≦A≦780 and B is 800≦B≦900.

[2] The method for manufacturing high strength steel sheets described in[1], wherein the chemical composition of the steel sheet furtherincludes one or more elements selected from B at 0.001 to 0.005%, Nb at0.005 to 0.05%, Ti at 0.005 to 0.05%, Cr at 0.001 to 1.0%, Mo at 0.05 to1.0%, Cu at 0.05 to 1.0% and Ni at 0.05 to 1.0% in terms of mass %.

[3] The method for manufacturing high strength steel sheets described in[1] or [2], further including, after the continuous annealing,electrolytically pickling the steel sheet in an aqueous solutioncontaining sulfuric acid.

[4] A high strength steel sheet manufactured by the method described inany of [1] to [3] in which a surface portion of the steel sheetextending from the steel sheet surface within a depth of 100 μm containsan oxide of at least one or more selected from Fe, Si, Mn, Al, P, B, Nb,Ti, Cr, Mo, Cu and Ni at 0.010 to 0.50 g/m² per single side surface, andin which with respect to a region extending from the steel sheet surfacewithin a depth of 10 μm, a crystalline Si/Mn oxide is present in grainsthat are within 1 μm from crystal grain boundaries of the steel sheet.

In the present invention, the term “high strength” means that thetensile strength TS is not less than 340 MPa. The high strength steelsheets in the invention include both cold rolled steel sheets and hotrolled steel sheets.

According to the present invention, a high strength steel sheet isobtained which exhibits excellent chemical convertibility and corrosionresistance after electrodeposition coating even in the case where thesteel sheet has a high Si content.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will be described in detail hereinbelow in termsof exemplary embodiments. In the following description, the unit for thecontents of individual elements in the chemical composition of steel is“mass %” and is indicated simply as “%” unless otherwise mentioned.

First, there will be described annealing atmosphere conditions that arethe most important requirement in the invention and determine thestructure of the surface of the steel sheet.

A chemical conversion treatment is performed after a steel sheet iscontinuously annealed in such a manner that the dew-point temperature ofthe atmosphere is controlled to become not less than −10° C. when theheating furnace inside temperature is in a limited range of not lessthan A° C. and not more than B° C. during the course of heating in anannealing furnace (A: 600≦A≦780, B: 800≦B≦900). In this manner, oxidesof easily oxidized elements (such as Si and Mn) are allowed to bepresent in appropriate amounts inside a surface portion of the steelsheet extending from the surface within a depth of 10 μm (hereinafter,such oxides will be referred to as internal oxides), thereby making itpossible to suppress selective surface oxidation of such elements as Siand Mn on the steel sheet surface that deteriorate the chemicalconvertibility of the steel after annealing (hereinafter, this oxidationwill be referred to as “surface segregation”) from occurring in thesurface portion of the steel sheet.

The lower limit temperature A is limited to be 600≦A≦780 for thefollowing reasons. When the temperature is in the range of less than600° C., the amount of surface segregation is inherently small. Thus, adeterioration in chemical convertibility is not caused in such atemperature range even if the dew-point temperature is not controlledand internal oxides are not formed. If the temperature is raised toabove 780° C. without controlling of the dew-point temperature, theamount of surface segregation is so increased that the inward diffusionof oxygen is inhibited and internal oxidation is unlikely to occur. Itis therefore necessary to control the dew-point temperature to becomenot less than −10° C. at least from when the temperature is in the rangeof not more than 780° C. Thus, the acceptable range of A is 600≦A≦780.For the above reason, it is preferable that A be a temperature as low aspossible within this range.

The upper limit temperature B is limited to be 800≦B≦900 for thefollowing reasons. The formation of internal oxides decreases the amountof easily oxidized elements (such as Si and Mn) present as solutesinside a surface portion of the steel sheet extending from the surfacewithin a depth of 10 μm (hereinafter, such a portion will be referred toas “deficient layer”), and thereby the easily oxidized elements aresuppressed from diffusing from the inside of steel toward the surface.In order to form such internal oxides as well as to form the deficientlayer enough to suppress the occurrence of surface segregation, thetemperature B needs to satisfy 800≦B≦900. If the temperature is lessthan 800° C., internal oxides are not formed sufficiently. If thetemperature exceeds 900° C., internal oxides are formed in excessivelylarge amounts and serve as starting points of a deterioration incorrosion resistance after electrodeposition coating.

The dew-point temperature is controlled to become not less than −10° C.when the temperature is in the range of not less than A° C. and not morethan B° C. for the following reasons. Increasing the dew-pointtemperature increases the potential of O₂ generated by the decompositionof H₂O, and therefore internal oxidation can be promoted. The amount offormed internal oxides is small if the dew-point temperature is in therange of below −10° C. The upper limit of the dew-point temperature isnot particularly limited. However, the amount of oxidation of ironincreases if the dew-point temperature is in excess of 90° C., causing arisk that annealing furnace walls or rollers may be degraded. Thus, thedew-point temperature is preferably not more than 90° C.

Next, the chemical composition of the high strength steel sheets ofinterest according to embodiments of the present invention will bedescribed.

C: 0.01 to 0.18%

Carbon increases workability by forming phases such as martensite in thesteel microstructure. In order to obtain this effect, carbon needs to becontained at not less than 0.01%. On the other hand, containing carbonin excess of 0.18% causes a decrease in elongation as well asdeteriorations in quality and weldability. Thus, the C content islimited to be not less than 0.01% and not more than 0.18%.

Si: 0.4 to 2.0%

Silicon increases the strength and the elongation of steel and istherefore an effective element for achieving a good quality. In order toobtain the objective strength in the present invention, silicon ispreferably contained at not less than 0.4%. Steel sheets having a Sicontent of less than 0.4% cannot achieve a strength of interest in theinvention and are substantially free of problems in terms of chemicalconvertibility. On the other hand, containing silicon in excess of 2.0%results in the saturation of steel strengthening effects as well as thesaturation of elongation enhancement, and achieving an improvement ofchemical convertibility becomes difficult. Thus, the Si content islimited to be not less than 0.4% and not more than 2.0%.

Mn: 1.0 to 3.0%

Manganese is an effective element for increasing the strength of steel.In order to ensure mechanical characteristics and strength, the Mncontent needs to be not less than 1.0%. On the other hand, containingmanganese in excess of 3.0% causes difficulties in ensuring weldabilityas well as in ensuring the balance between strength and ductility. Thus,the Mn content is limited to be not less than 1.0% and not more than3.0%.

Al: 0.001 to 1.0%

Aluminum is added for the purpose of deoxidation of molten steel. Thedeoxidation effect for molten steel is obtained by adding aluminum atnot less than 0.001%. On the other hand, adding aluminum in excess of1.0% increases costs and further results in an increase in the amount ofsurface segregation of aluminum, thereby making it difficult to improvechemical convertibility. Thus, the Al content is limited to be not lessthan 0.001% and not more than 1.0%.

P: 0.005 to not more than 0.060%

Phosphorus is one of elements that are inevitably present in steel. Anincrease in cost is expected if the P content is reduced to below0.005%. Thus, the P content is specified to be not less than 0.005%. Onthe other hand, any P content exceeding 0.060% leads to a decrease inweldability and causes a marked deterioration in chemical convertibilityto such an extent that it becomes difficult to improve chemicalconvertibility even by the present invention. Thus, the P content islimited to be not less than 0.005% and not more than 0.060%.

S: S≦0.01%

Sulfur is one of inevitable elements. The lower limit is notparticularly limited. However, the presence of this element in a largeamount causes decreases in weldability and corrosion resistance. Thus,the S content is limited to be not more than 0.01%.

In order to control the balance between strength and ductility, one ormore elements selected from 0.001 to 0.005% of B, 0.005 to 0.05% of Nb,0.005 to 0.05% of Ti, 0.001 to 1.0% of Cr, 0.05 to 1.0% of Mo, 0.05 to1.0% of Cu and 0.05 to 1.0% of Ni may be added as required.

The appropriate amounts of these optional elements are limited for thefollowing reasons.

B: 0.001 to 0.005%

The effect in promoting hardening is hardly obtained if the B content isless than 0.001%. On the other hand, adding boron in excess of 0.005%results in a decrease in chemical convertibility. Thus, when boron iscontained, the B content is limited to be not less than 0.001% and notmore than 0.005%.

Nb: 0.005 to 0.05%

The effect in adjusting strength is hardly obtained if the Nb content isless than 0.005%. On the other hand, containing niobium in excess of0.05% results in an increase in cost. Thus, when niobium is contained,the Nb content is limited to be not less than 0.005% and not more than0.05%.

Ti: 0.005 to 0.05%

The effect in adjusting strength is hardly obtained if the Ti content isless than 0.005%. On the other hand, containing titanium in excess of0.05% results in a decrease in chemical convertibility. Thus, whentitanium is contained, the Ti content is limited to be not less than0.005% and not more than 0.05%.

Cr: 0.001 to 1.0%

The effect in promoting hardening is hardly obtained if the Cr contentis less than 0.001%. On the other hand, containing chromium in excess of1.0% results in the surface segregation of chromium and a consequentdecrease in weldability. Thus, when chromium is contained, the Crcontent is limited to be not less than 0.001% and not more than 1.0%.

Mo: 0.05 to 1.0%

The effect in adjusting strength is hardly obtained if the Mo content isless than 0.05%. On the other hand, containing molybdenum in excess of1.0% results in an increase in cost. Thus, when molybdenum is contained,the Mo content is limited to be not less than 0.05% and not more than1.0%.

Cu: 0.05 to 1.0%

The effect in promoting the formation of a retained γ-phaseis hardlyobtained if the Cu content is less than 0.05%. On the other hand,containing copper in excess of 1.0% results in an increase in cost.Thus, when copper is contained, the Cu content is limited to be not lessthan 0.05% and not more than 1.0%.

Ni: 0.05 to 1.0%

The effect in promoting the formation of a retained phase is hardlyobtained if the Ni content is less than 0.05%. On the other hand,containing nickel in excess of 1.0% results in an increase in cost.Thus, when nickel is contained, the Ni content is limited to be not lessthan 0.05% and not more than 1.0%.

The balance after the deduction of the aforementioned elements isrepresented by Fe and inevitable impurities.

Next, there will be described an embodiment of a method formanufacturing the high strength steel sheets according to the inventionas well as the reasons why the conditions in the method are limited. Forexample, a steel having the above-described chemical composition is hotrolled and is thereafter cold rolled, and subsequently the steel sheetis annealed in a continuous annealing facility and is subjected to achemical conversion treatment. Here, in the present invention, theannealing is carried out in such a manner that the dew-point temperatureof the atmosphere is controlled to become not less than −10° C. when theheating furnace inside temperature is in the range of not less than A°C. and not more than 3° C. during the course of heating (A: 600≦A≦780,B: 800≦B≦900). This is the most important aspect in the invention. Bycontrolling the dew-point temperature, namely, the oxygen partialpressure in the atmosphere during the annealing step, the oxygenpotential is increased with the result that easily oxidized elementssuch as Si and Mn are internally oxidized beforehand immediately beforea chemical conversion treatment and the activities of Si and Mn in thesurface portion of the steel sheet are lowered. Consequently, theexternal oxidation of these elements is suppressed, resulting in animprovement in chemical convertibility. In the above processing ofsteel, it is possible to anneal the hot rolled steel sheet withoutsubjecting it to cold rolling.

Hot Rolling

Hot rolling may be performed under usual conditions.

Pickling

It is preferable to perform a pickling treatment after hot rolling. Inthe pickling step, black scales formed on the surface are removed andthe steel sheet is subjected to cold rolling. Pickling conditions arenot particularly limited.

Cold Rolling

Cold rolling is preferably carried out with a draft of not less than 40%and not more than 80%. If the draft is less than 40%, therecrystallization temperature becomes lower and the steel sheet tends tobe deteriorated in mechanical characteristics. On the other hand,because the steel sheet of the invention is a high strength steel sheet,cold rolling the steel sheet with a draft exceeding 80% increases notonly the rolling costs but also the amount of surface segregation duringannealing, possibly resulting in a decrease in chemical convertibility.

The steel sheet that has been cold rolled or hot rolled is annealed andthen subjected to a chemical conversion treatment.

In an annealing furnace, the steel sheet undergoes a heating step inwhich the steel sheet is heated to a predetermined temperature in anupstream heating zone and a soaking step in which the steel sheet isheld in a downstream soaking zone at a predetermined temperature for aprescribed time. Next, a cooling step is performed.

As described above, the annealing is carried out in such a manner thatthe dew-point temperature of the atmosphere is controlled to become notless than −10° C. when the heating furnace inside temperature is in therange of not less than A° C. and not more than B° C. (A: 600≦A≦780, B:800≦B≦900). Except when the temperature is in the range of not less thanA° C. and not more than B° C., the dew-point temperature of theatmosphere in the annealing furnace is not particularly limited, but ispreferably in the range of −50° C. to −10° C.

The gas components in the annealing furnace include nitrogen, hydrogenand inevitable impurities. Other gas components may be present as longas they are not detrimental in achieving the advantageous effects of theinvention. If the hydrogen concentration in the annealing furnaceatmosphere is less than 1 volt, the activation effect by reductioncannot be obtained and chemical convertibility is deteriorated. Althoughthe upper limit is not particularly limited, costs are increased and theeffect is saturated if the hydrogen concentration exceeds 50 volt. Thus,the hydrogen concentration is preferably not less than 1 vol % and notmore than 50 vol %. The gas components in the annealing furnace excepthydrogen gas are nitrogen gas and inevitable impurity gases. Other gascomponents may be present as long as they are not detrimental inachieving the advantageous effects of the invention.

After the steel sheet is cooled from the temperature range of not lessthan 750° C., hardening and tempering may be performed as required.Although the conditions for these treatments are not particularlylimited, it is desirable that tempering be performed at a temperature of150 to 400° C. The reasons are because elongation tends to bedeteriorated if the temperature is less than 150° C. as well as becausehardness tends to be decreased if the temperature is in excess of 400°C.

According to the present invention, good chemical convertibility can beensured even without performing electrolytic pickling. However, it ispreferable that electrolytic pickling be performed in order to removetrace amounts of oxides that have been inevitably generated by surfacesegregation during annealing and thereby to ensure better chemicalconvertibility.

The electrolytic pickling conditions are not particularly limited.However, in order to efficiently remove the inevitably formed surfacesegregation of silicon and manganese oxides formed during the annealing,alternating electrolysis at a current density of not less than 1 A/dm²is desirable. The reasons why alternating electrolysis is selected arebecause the pickling effects are low if the steel sheet is fixed to acathode as well as because if the steel sheet is fixed to an anode, ironthat is dissolved during electrolysis is accumulated in the picklingsolution and the Fe concentration in the pickling solution is increasedwith the result that the attachment of iron to the surface of the steelsheet causes problems such as dry contamination.

The pickling solution used in the electrolytic pickling is notparticularly limited. However, nitric acid or hydrofluoric acid is notpreferable because they are highly corrosive to a facility and requirecareful handling. Hydrochloric acid is not preferable because chlorinegas can be generated from the cathode. In view of corrosiveness andenvironment, the use of sulfuric acid is preferable. The sulfuric acidconcentration is preferably not less than 5 mass % and not more than 20mass %. If the sulfuric acid concentration is less than 5 mass %, theconductivity is so lowered that the bath voltage is raised duringelectrolysis possibly to increase the power load. On the other hand, anysulfuric acid concentration exceeding 20 mass % leads to a cost problembecause a large loss is caused due to drag-out.

The temperature of the electrolytic solution is preferably not less than40° C. and not more than 70° C. Because the bath temperature is raisedby the generation of heat by continuous electrolysis, the picklingeffect may be lowered if the temperature is less than 40° C. Further,maintaining the temperature below 40° C. is sometimes difficult.Furthermore, a temperature exceeding 70° C. is not preferable in view ofthe durability of the lining of the electrolytic cell.

The high strength steel sheets of the present invention are obtained inthe above manner.

As a result, the inventive steel sheet has a characteristic structure ofthe surface described below.

A surface portion of the steel sheet extending from the steel sheetsurface within a depth of 100 μm contains an oxide of one or moreselected from Fe, Si, Mn, Al and P, as well as from B, Nb, Ti, Cr, Mo,Cu and Ni at a total amount of 0.010 to 0.50 g/m² per single sidesurface. Further, with respect to a region extending from the steelsheet surface to a depth of 10 μm, a crystalline Si/Mn complex oxide ispresent in base iron grains that are within 1 μm from grain boundaries.

In a high strength steel sheet containing Si and a large amount of Mn,more sophisticated controlling of the microstructure and configurationof a surface portion of the steel sheet which can be an origin ofcorrosion or cracks is necessary in order to achieve satisfactorycorrosion resistance after electrodeposition coating. For the purpose ofensuring chemical convertibility, the present invention first providesthat the dew-point temperature is controlled as described hereinabove inorder to increase the oxygen potential in the annealing step. As aresult of the oxygen potential being increased, easily oxidized elementssuch as Si and Mn are internally oxidized beforehand immediately beforea chemical conversion treatment and the activities of Si and Mn in thesurface portion of the steel sheet are lowered. Consequently, theexternal oxidation of these elements is suppressed, resulting inimprovements in chemical convertibility and corrosion resistance afterelectrodeposition coating. These improvements are obtained byconfiguring the steel sheet such that the surface portion of the steelsheet extending from the steel sheet surface within a depth of 100 μmcontains an oxide of at least one or more selected from Fe, Si, Mn, Aland P, as well as from B, Nb, Ti, Cr, Mo, Cu and Ni at not less than0.010 g/m² per single side surface. The effects are saturated even whensuch oxides are present in excess of 0.50 g/m². Thus, the upper limit isspecified to be 0.50 g/m².

In the case where internal oxides are present only at grain boundariesand not in grains, the intergranular diffusion of easily oxidizedelements in steel can be suppressed but the intragranular diffusionthereof may not be suppressed sufficiently. Thus, as describedhereinabove, the present invention provides that internal oxidation iscaused to take place not only at grain boundaries' but also in grains bycontrolling the dew-point temperature of the atmosphere to become notless than −10° C. when the heating furnace inside temperature is in therange of not less than A° C. and not more than B° C. (A: 600≦A≦780, B:800≦B≦900). In detail, a crystalline Si/Mn complex oxide is caused to bepresent in base iron grains that are within 1 μm from grain boundariesin a region extending from the steel sheet surface to a depth of 10 μm.Because of the oxide being present in base iron grains, the amount ofsolute silicon and manganese in base iron grains in the vicinity of theoxide is decreased. As a result, the surface segregation of Si and Mndue to intragranular diffusion can be suppressed.

The structure of the surface of the high strength steel sheet obtainedby the manufacturing method according to the present invention is asdescribed above. There is no problem even when the oxides have beengrown so as to extend to a region that is more than 100 μm away from thesteel sheet surface. Further, no problems are caused even when thecrystalline Si/Mn complex oxide is caused to be present in base irongrains that are more than 1 μm away from grain boundaries in a regionextending from the steel sheet surface to a depth in excess of 10 μm.

Example 1

Hereinbelow, the present invention will be described in detail based onEXAMPLES.

Hot rolled steel sheets with a steel composition described in Table 1were pickled to remove black scales and were thereafter cold rolled togive cold rolled steel sheets with a thickness of 1.0 mm. Cold rollingwas omitted for some of the steel sheets. That is, as-descaled hotrolled steel sheets (thickness: 2.0 mm) were also provided.

TABLE 1 (mass %) Steel code C S i Mn Al P S Cr Mo B Nb Cu Ni Ti A 0.040.1 1.9 0.04 0.01 0.003 — — — — — — — B 0.03 0.4 2.0 0.04 0.01 0.003 — —— — — — — C 0.09 0.9 2.1 0.03 0.01 0.004 — — — — — — — D 0.13 1.3 2.00.03 0.01 0.003 — — — — — — — E 0.09 1.7 1.9 0.03 0.01 0.003 — — — — — —— F 0.08 2.0 2.1 0.03 0.01 0.003 — — — — — — — G 0.11 1.3 2.8 0.04 0.010.003 — — — — — — — H 0.12 1.3 2.0 0.95 0.01 0.003 — — — — — — — I 0.121.3 2.0 0.04 0.06 0.004 — — — — — — — J 0.12 1.3 2.1 0.03 0.01 0.008 — —— — — — — K 0.12 1.3 1.9 0.02 0.01 0.003 0.7 — — — — — — L 0.12 1.3 2.00.04 0.01 0.003 — 0.12 — — — — — M 0.12 1.3 2.1 0.03 0.01 0.003 — —0.005 — — — — N 0.12 1.3 2.0 0.05 0.01 0.003 — — 0.001 0.04 — — — O 0.121.3 1.9 0.03 0.01 0.004 — 0.11 — — 0.2 0.3 — P 0.12 1.3 1.9 0.04 0.010.003 — — 0.003 — — — 0.03 Q 0.12 1.3 2.0 0.03 0.01 0.004 — — — — — —0.05 R 0.20 1.3 2.1 0.04 0.01 0.003 — — — — — — — S 0.12 2.1 1.9 0.040.01 0.003 — — — — — — — T 0.12 1.3 3.1 0.04 0.01 0.004 — — — — — — — U0.12 1.3 2.0 1.10 0.01 0.004 — — — — — — — V 0.12 1.3 1.9 0.03 0.070.003 — — — — — — — W 0.12 1.3 2.1 0.04 0.01 0.015 — — — — — — —Underlines indicate “outside the inventive range”.

Next, the cold rolled steel sheets and the hot rolled steel sheetsobtained above were introduced into a continuous annealing facility. Thesteel sheet was passed through the annealing facility while controllingthe heating furnace inside temperature and the dew-point temperature asdescribed in. Table 2. The annealed steel sheet was thereafter subjectedto water hardening and then to tempering at 300° C. for 140 seconds.Subsequently, electrolytic pickling was performed by alternatingelectrolysis in a 5 mass % aqueous sulfuric acid solution at 40° C.under current density conditions described in Table 2 while switchingthe polarity of the sample sheet between anodic and cathodic alternatelyeach after 3 seconds. Thus, sample sheets were prepared. The dew-pointtemperature in the annealing furnace was basically set at −35° C. exceptwhen the dew-point temperature was controlled as described above. Thegas components in the atmosphere included nitrogen gas, hydrogen gas andinevitable impurity gases. The dew-point temperature was controlled bydehumidifying the atmosphere or by removing water in the atmosphere byabsorption. The hydrogen concentration in the atmosphere was basicallyset at 10 vol %.

With respect to the obtained sample sheets, TS and El were measured inaccordance with a tensile testing method for metallic materialsdescribed in JIS Z 2241. Further, the sample sheets were tested toexamine chemical convertibility and corrosion resistance, as well as theamount of oxides present in a surface portion of the steel sheetextending immediately from the surface of the steel sheet to a depth of100 μm (the internal oxidation amount). The measurement methods and theevaluation criteria are described below.

Chemical Convertibility

Chemical convertibility was evaluated by the following method.

A chemical conversion treatment liquid (PALBOND L3080 (registeredtrademark)) manufactured by Nihon Parkerizing Co., Ltd. was used. Achemical conversion treatment was carried out in the following manner.

The sample sheet was degreased with degreasing liquid FINE CLEANER(registered trademark) manufactured by Nihon Parkerizing Co., Ltd., andwas thereafter washed with water. Subsequently, the surface of thesample sheet was conditioned for 30 seconds with surface conditioningliquid PREPAREN Z (registered trademark) manufactured by NihonParkerizing Co., Ltd. The sample sheet was then soaked in the chemicalconversion treatment liquid (PALBOND L3080) at 43° C. for 120 seconds,washed with water and dried with hot air.

The sample sheet after the chemical conversion treatment was observedwith a scanning electron microscope (SEM) at 500× magnification withrespect to randomly selected five fields of view. The area ratio of theregions that had not been covered with the chemical conversion coatingwas measured by image processing. Chemical convertibility was evaluatedbased on the area ratio of the non-covered regions according to thefollowing criteria. The symbol 0 indicates an acceptable level.

◯: not more than 10%

x: more than 10%

Corrosion Resistance after Electrodeposition Coating

A 70 mm×150 mm test piece was cut out from the sample sheet that hadbeen subjected to the above chemical conversion treatment. The testpiece was cationically electrodeposition coated with PN-150G (registeredtrademark) manufactured by NIPPON PAINT Co., Ltd. (baking conditions:170° C.×20 min, film thickness: 25 μm). Thereafter, the edges and thenon-test surface were sealed with an Al tape, and the test surface wascut deep into the base steel with a cutter knife to create a cross cutpattern (cross angle: 60°), thereby preparing a sample.

Next, the sample was soaked in a 5 mass % aqueous NaCl solution (55° C.)for 240 hours, removed from the solution, washed with water and dried.Thereafter, an adhesive tape was applied to the cross cut pattern andwas peeled therefrom. The exfoliation width was measured and wasevaluated based on the following criteria. The symbol ◯ indicates anacceptable level.

◯: The exfoliation width from each cut line was less than 2.5 mm.

x: The exfoliation width from each cut line was 2.5 mm or more.

Workability

To evaluate workability, a JIS No. 5 tensile test piece was sampled fromthe sample sheet in a direction that was 90° relative to the rollingdirection. The test piece was subjected to a tensile test at a constantcross head speed of 10 mm/min in accordance with JIS Z 2241, therebydetermining the tensile strength (TS/MPa) and the elongation (El %). Forsteel sheets with TS of less than 650 MPa, workability was evaluated tobe good when TS×El≧22000 and to be bad when TS×El<22000. For steelsheets with TS of 650 MPa to 900 MPa, workability was evaluated to begood when TS×El≧20000 and to be bad when TS×El<20000. For steel sheetswith TS of not less than 900 MPa, workability was evaluated to be goodwhen TS×El≧18000 and to be bad when TS×El<18000.

Internal Oxidation Amount in Region from Steel Sheet Surface to Depth of100 μm

The internal oxidation amount was measured by an “impulse furnacefusion-infrared absorption method”. It should be noted that the amountof oxygen present in the starting material (namely, the high strengthsteel sheet before annealing) should be subtracted. Thus, in theinvention, surface portions on both sides of the continuously annealedhigh strength steel sheet were polished by at least 100 μm andthereafter the oxygen concentration in the steel was measured. Themeasured value was obtained as the oxygen amount OH of the startingmaterial. Further, the oxygen concentration was measured across theentirety of the continuously annealed high strength steel sheet in thesheet thickness direction. The measured value was obtained as the oxygenamount OI after internal oxidation. The difference between OI and OH(=OI−OH) was calculated wherein OI was the oxygen amount in the highstrength steel sheet after internal oxidation and OH was the oxygenamount in the starting material. The difference was then converted to anamount per unit area (namely, 1 m²) on one surface, thereby determiningthe internal oxidation amount (g/m²).

The results and the manufacturing conditions are described in Table 2.

TABLE 2 Internal oxide in region from immediately under coating to depthof 10 μm Annealing furnace Internal oxidation Presence of Dew-pointamount (g/m²) in intragranular temp. (° C.) region from precipitateSteel at between Maximum immediately immediately under Steel Si Mn SteelTemp. A Temp. B temp. A temp. under coating to coating at depth withinNo. code (mass %) (mass %) sheet (° C.) (° C.) and temp. B (° C.) depthof 100 μm Presence 1 um from grain 1 D 1.3 2.0 Cold 600 700 −5 850 0.004X X rolled 2 D 1.3 2.0 Cold 600 790 −5 850 0.009 X X rolled 3 D 1.3 2.0Cold 600 800 −5 800 0.021 ◯ ◯ rolled 4 D 1.3 2.0 Cold 600 800 −5 8300.025 ◯ ◯ rolled 5 D 1.3 2.0 Cold 600 800 −5 860 0.028 ◯ ◯ rolled 6 D1.3 2.0 Cold 600 800 −5 890 0.033 ◯ ◯ rolled 7 D 1.3 2.0 Cold 650 850 −5850 0.022 ◯ ◯ rolled 8 D 1.3 2.0 Cold 700 850 −5 850 0.020 ◯ ◯ rolled 9D 1.3 2.0 Hot 700 850 −5 850 0.123 ◯ ◯ rolled 10 D 1.3 2.0 Cold 750 850−5 850 0.015 ◯ ◯ rolled 11 D 1.3 2.0 Cold 780 850 −5 850 0.012 ◯ ◯rolled 12 D 1.3 2.0 Cold 790 850 −5 850 0.007 X X rolled 13 D 1.3 2.0Cold 700 850 −35 850 0.006 X X rolled 14 D 1.3 2.0 Cold 700 850 −15 8500.008 X X rolled 15 D 1.3 2.0 Cold 700 850 −10 850 0.011 ◯ ◯ rolled 16 D1.3 2.0 Cold 700 850 0 850 0.068 ◯ ◯ rolled 17 D 1.3 2.0 Cold 700 850 20850 0.221 ◯ ◯ rolled 18 D 1.3 2.0 Cold 700 850 60 850 0.436 ◯ ◯ rolled19 D 1.3 2.0 Cold 700 850 −5 850 0.021 ◯ ◯ rolled 20 D 1.3 2.0 Cold 700850 −5 850 0.019 ◯ ◯ rolled 21 D 1.3 2.0 Cold 700 850 −5 850 0.020 ◯ ◯rolled 22 A 0.1 1.9 Cold 700 850 −5 850 0.021 ◯ ◯ rolled 23 B 0.4 2.0Cold 700 850 −5 850 0.009 ◯ ◯ rolled 24 C 0.9 2.1 Cold 700 850 −5 8500.011 ◯ ◯ rolled 25 E 1.7 1.9 Cold 700 850 −5 850 0.030 ◯ ◯ rolled 26 F2.0 2.1 Cold 700 850 −5 850 0.039 ◯ ◯ rolled 27 G 1.3 2.8 Cold 700 850−5 850 0.021 ◯ ◯ rolled 28 H 1.3 2.0 Cold 700 850 −5 850 0.051 ◯ ◯rolled 29 I 1.3 2.0 Cold 700 850 −5 850 0.022 ◯ ◯ rolled 30 J 1.3 2.1Cold 700 850 −5 850 0.015 ◯ ◯ rolled 31 K 1.3 1.9 Cold 700 850 −5 8500.016 ◯ ◯ rolled 32 L 1.3 2.0 Cold 700 850 −5 850 0.013 ◯ ◯ rolled 33 M1.3 2.1 Cold 700 850 −5 850 0.014 ◯ ◯ rolled 34 N 1.3 2.0 Cold 700 850−5 850 0.016 ◯ ◯ rolled 35 O 1.3 1.9 Cold 700 850 −5 850 0.015 ◯ ◯rolled 36 P 1.3 1.9 Cold 700 850 −5 850 0.013 ◯ ◯ rolled 37 Q 1.3 2.0Cold 700 850 −5 850 0.017 ◯ ◯ rolled 38 R 1.3 2.1 Cold 700 850 −5 8500.019 ◯ ◯ rolled 39 S 2.1 1.9 Cold 700 850 −5 850 0.052 ◯ ◯ rolled 40 T1.3 3.1 Cold 700 850 −5 850 0.016 ◯ ◯ rolled 41 U 1.3 2.0 Cold 700 850−5 850 0.051 ◯ ◯ rolled 42 V 1.3 1.9 Cold 700 850 −5 850 0.033 ◯ ◯rolled 43 W 1.3 2.1 Cold 700 850 −5 850 0.020 ◯ ◯ rolled Corrosionresistance after Electrolytic Current density Chemical electrodepositionTS No. pickling A/dm² convertibility coating MPa El % TS × ElWorkability Remarks  1 Not — X X 1051 20.8 21861 Good COMP. EX.performed  2 Not — X X 1029 21.1 21712 Good COMP. EX. performed  3 Not —◯ ◯ 1031 20.4 21032 Good INV. EX. performed  4 Not — ◯ ◯ 1025 20.3 20808Good INV. EX. performed  5 Not — ◯ ◯ 1021 20.2 20624 Good INV. EX.performed  6 Not — ◯ ◯ 1029 20.0 20580 Good INV. EX. performed  7 Not —◯ ◯ 1034 20.7 21404 Good INV. EX. performed  8 Not — ◯ ◯ 1039 20.6 21403Good INV. EX. performed  9 Not — ◯ ◯ 1040 20.3 21112 Good INV. EX.performed 10 Not — ◯ ◯ 1024 20.4 20890 Good INV. EX. performed 11 Not —◯ ◯ 1031 20.8 21445 Good INV. EX. performed 12 Not — X X 990 20.9 20691Good COMP. EX. performed 13 Not — X X 991 20.7 20514 Good COMP. EX.performed 14 Not — X X 1159 18.0 20862 Good COMP. EX. performed 15 Not —◯ ◯ 1044 19.9 20776 Good INV. EX. performed 16 Not — ◯ ◯ 1033 20.4 21073Good INV. EX. performed 17 Not — ◯ ◯ 1050 20.6 21630 Good INV. EX.performed 18 Not — ◯ ◯ 1051 20.1 21125 Good INV. EX. performed 19Performed 1 ◯ ◯ 1041 20.0 20820 Good INV. EX. 20 Performed 5 ◯ ◯ 104220.7 21569 Good INV. EX. 21 Performed 10  ◯ ◯ 1044 20.9 21820 Good INV.EX. 22 Not — ◯ ◯ 712 26.5 18868 Bad COMP. EX. performed 23 Not — ◯ ◯1010 20.9 21109 Good INV. EX. performed 24 Not — ◯ ◯ 1021 21.4 21849Good INV. EX. performed 25 Not — ◯ ◯ 1036 22.8 23621 Good INV. EX.performed 26 Not — ◯ ◯ 1029 20.5 21095 Good INV. EX. performed 27 Not —◯ ◯ 1064 19.7 20961 Good INV. EX. performed 28 Not — ◯ ◯ 1066 20.3 21640Good INV. EX. performed 29 Not — ◯ ◯ 1145 20.1 23015 Good INV. EX.performed 30 Not — ◯ ◯ 1044 19.9 20776 Good INV. EX. performed 31 Not —◯ ◯ 1063 19.4 20622 Good INV. EX. performed 32 Not — ◯ ◯ 1052 19.5 20514Good INV. EX. performed 33 Not — ◯ ◯ 1037 20.9 21673 Good INV. EX.performed 34 Not — ◯ ◯ 1077 20.4 21971 Good INV. EX. performed 35 Not —◯ ◯ 1078 21.0 22638 Good INV. EX. performed 36 Not — ◯ ◯ 811 26.7 21654Good INV. EX. performed 37 Not — ◯ ◯ 1055 19.7 20784 Good INV. EX.performed 38 Not — ◯ ◯ 1066 12.8 13645 Bad COMP. EX. performed 39 Not —X ◯ 1212 16.4 19877 Good COMP. EX. performed 40 Not — ◯ ◯ 1125 13.415075 Bad COMP. EX. performed 41 Not — X X 1079 21.4 23091 Good COMP.EX. performed 42 Not — X ◯ 1144 19.4 22194 Good COMP. EX. performed 43Not — ◯ X 1079 20.3 21904 Good COMP. EX. performed Underlines indicatethat manufacturing conditions are outside the inventive ranges.

From Table 2, the high strength steel sheets manufactured by theinventive method were shown to be excellent in chemical convertibility,corrosion resistance after electrodeposition coating and workability inspite of the fact that these high strength steel sheets contained largeamounts of easily oxidized elements such as Si and Mn.

On the other hand, the steel sheets obtained in COMPARATIVE EXAMPLESwere poor in at least one of chemical convertibility, corrosionresistance after electrodeposition coating and workability.

The high strength steel sheets according to the present invention areexcellent in chemical convertibility, corrosion resistance andworkability, and can be used as surface-treated steel sheets forreducing the weight and increasing the strength of bodies ofautomobiles. Besides automobiles, the inventive high strength steelsheets can be used as surface-treated steel sheets having corrosionresistance on the base steel sheet in a wide range of applicationsincluding home appliances and building materials.

1. A method for manufacturing high strength steel sheets, comprisingcontinuous annealing of a steel sheet which includes, in terms of mass%, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001to 1.0%, P at 0.005 to 0.060% and S at ≦0.01%, the balance beingrepresented by Fe and inevitable impurities, while controlling thedew-point temperature of the atmosphere to become not less than −10° C.when the heating furnace inside temperature is in the range of not lessthan A° C. and not more than B° C. during the course of heating whereinA is 600≦A≦780 and B is 800≦B≦900.
 2. The method for manufacturing highstrength steel sheets according to claim 1, wherein the chemicalcomposition of the steel sheet further includes one or more elementsselected from B at 0.001 to 0.005%, Nb at 0.005 to 0.05%, Ti at 0.005 to0.05%, Cr at 0.001 to 1.0%, Mo at 0.05 to 1.0%, Cu at 0.05 to 1.0% andNi at 0.05 to 1.0% in terms of mass %.
 3. The method for manufacturinghigh strength steel sheets according to claim 1, further comprising,after the continuous annealing, electrolytically pickling the steelsheet in an aqueous solution containing sulfuric acid.
 4. A highstrength steel sheet manufactured by the method described in claim 1 inwhich a surface portion of the steel sheet extending from the steelsheet surface within a depth of 100 μm contains an oxide of at least oneor more selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu and Ni at0.010 to 0.50 g/m² per single side surface, and in which with respect toa region extending from the steel sheet surface within a depth of 10 μm,a crystalline Si/Mn oxide is present in grains that are within 1 μm fromcrystal grain boundaries of the steel sheet.